Matrix-assisted laser desorption and ionization mass spectrometry (MALDI-MS) is an important analytical tool for the study and identification of biomolecules, particularly proteins, peptides, and nucleic acids such as DNA and RNA.
MALDI-MS results in a mass spectrum that graphically identifies biomolecules according to peaks that correspond to the biomolecules' concentration and mass. Using a library of known peaks, the biomolecules can be identified.
Various methods exist for the preparation of samples for analysis by MALDI-MS, including the dried droplet method. In the dried droplet method an aqueous sample containing the subject biomolecule is mixed with an organic compound, the matrix, which is usually suspended in an easily evaporative aqueous-organic solvent. The resulting liquid mixture containing the biomolecule, the matrix, aqueous solution, and solvents is referred to herein as the specimen.
The specimen is applied to the sample plate in a predetermined target area and allowed to dry. As the solvent begins to evaporate, and the biomolecule and matrix become more concentrated, the matrix molecules crystallize from solution while drying on the sample plate. The resulting crystals entrap the biomolecules on and/or within the crystals and in due course deposit on the sample plate.
Other methods of applying the specimen to the sample plate are also known. In the electrospray deposition method, the specimen is applied to the sample plate as a fine mist of microdroplets that evaporates very quickly forming the specimen crystals.
To analyze the biomolecules, the sample plate is inserted into the sampling compartment of a mass spectrometry instrument. A voltage is applied to the sample plate to permit the flow of electric current over the sample plate and prevent the possibility of an electrical charge buildup. To desorb the crystals, an ultra-violet (UV) laser scans the target area either by manual direction or in a predetermined automated fashion to irradiate the crystals. The laser beam radiation is absorbed by the matrix molecules, resulting in a vaporization of both the matrix molecules and the biomolecules. Once in the vapor phase, while still in close proximity to the target area, a charge transfer occurs as the matrix molecule loses a proton to the biomolecule. The ionized biomolecules are then drawn into the mass spectrometer where they are analyzed. Data processing yields a mass spectrum of a series of characteristic peaks corresponding to the biomolecules and matrix molecules. The signature of peaks is used to identify the biomolecules by reference to known peaks.
Prior art of interest includes U.S. Pat. No. 6,287,872 (herein incorporated by reference) relating to sample support plates for the mass spectrometric analysis of large molecules, such as biomolecules, methods for the manufacture of such sample support plates and methods for loading the sample support plates with samples of biomolecules from solutions together with matrix substance for the ionization of the biomolecules using matrix-assisted laser desorption (MALDI).
Also of interest is U.S. Pat. No. 5,958,345 (herein incorporated by reference) relating to a sample support for holding samples for use with an analysis instrument. The sample support is for use with analysis instruments, which rely on a beam of radiation or accelerated particles and a method for making the same. The holder includes a frame with one or more orifices covered by a support surface, typically in the form of a thin polymer film. The film is divided into hydrophobic and hydrophilic portions to isolate precise positions where samples can be placed to intersect a probe beam during analysis.
MALDI-MS performance suffers chiefly from analysis insensitivity. The sample plates that are used in MALDI-MS are typically metallic plates due to the need to apply a voltage across the plate. Known trays have a smooth hydrophilic surface where the applied specimen drop spreads over a relatively large area before drying and forming crystals. Consequently, to effectively irradiate the crystals the UV laser has to scan this enlarged area requiring extra time.
Another drawback of metallic plates is that they unfortunately often provide unsuitable results due to unintentional contamination from detergents. Since, metallic plates are also expensive, they are used repeatedly. Washing between each use may contaminate subsequent analysis.
It is known, that specimens are non-homogeneously distributed on and/or within the lattice that located at the specimen periphery. It is further known that some of these matrix crystals bear more biomolecules than others. Thus, as the laser covers a likely search area at the specimen periphery, it scans “sweet spots” having a comparatively higher specimen concentration in the matrices. When irradiated and detected, the sweet spots provide an inaccurate concentration reading of the biomolecule.
What is desired, therefore, is a sample plate for MALDI-MS analysis of a specimen wherein crystals are located in a sample site.
What is also desired is a durable and cost effective sample plate which enables archiving of samples.
What is also desired is a rough surface that is hydrophobic to enhance the formation of crystals in a sample site. What is further desired is a higher ratio of surface area to planar area of the hydrophobic mask.